Field of the Invention
This invention relates to a circuit for controlling beam current in am image pick-up tube, and more particularly to a circuit for controlling beam current in an image pick-up tube in which the electron beam thereof is cut off or blanked during horizontal and vertical retrace intervals.
It is well-known in the television art to cut off or blank the electron beam in an image pick-up tube during horizontal and vertical retrace intervals. Typically, this is accomplished by applying a high positive bias to the cathode electrode and/or a high negative bias to the control grid electrode of the pick-up tube.
Many arrangements are known in the art for blanking the beam current during periodic intervals. One prior art beam blanking circuit employs a series-connected resistor and transistor, the junction of which is connected to a cathode electrode of the pick-up tube. However, in such a device, a DC current path is formed through the series-connected resistor and transistor during each retrace interval as well as during the beam scanning interval. Therefore, there is a relatively large power consumption which is deleterious for battery operated portable cameras. Further, in detecting a cathode current for controlling a beam current in the prior art circuit, additional resistors are required.